In the actual movie, though, the saucer is moving cleanly away until the shockwave hits it and sends it flying towards Veridian III (the saucer is actually seen being knocked nose down). With the helm dead, there's nothing anyone sitting at the helm could have done to prevent the ship from crashing.

After the shockwave hit? No. It's no secret that exploding things create shockwaves. We'd already seen that once in this movie alone up to this point. It wasn't a directional or curved shockwave or anything weird like that, but a symmetrical expanding sphere.

The only way that expanding sphere shock wave would push them into the planet is if the saucer was between the planet and the drive. The helm didn't die until -after- it hit. So while they were separating Troi could have very easily navigated so the drive section, which everyone knew was going to explode, was between the planet and the saucer. Even as they separated Troi was turning, indicating she had full helm control at the time. A turn in the opposite direction would've kept the saucer a going concern.